Transistors in which a semiconductor material is deposited over a substrate having an insulating surface to be used as an active layer (hereinafter referred to as deposited-film transistors) have been researched. A silicon-based semiconductor material such as amorphous silicon has been conventionally used as an active layer; however, attention is focused in recent years on research on transistors in which an oxide semiconductor material is used as an active layer. This is because a transistor in which an oxide semiconductor material is used as an active layer (hereinafter referred to as an oxide semiconductor transistor) has higher on-state current and lower off-state current than a transistor in which amorphous silicon is used as an active layer.
Further, there have been attempts to develop a semiconductor device which functions as a memory or the like by forming an oxide semiconductor transistor having the above features in a layer different from a transistor including single crystal silicon, and the like (Patent Document 1 and Non-Patent Document 1). In the structure of such a semiconductor device, the transistor in the upper layer is preferably a bottom-gate transistor. This is because a wiring which electrically connects the transistor in the lower layer can serve also as a gate electrode of the transistor in the upper layer.